Capillary electrophoresis is a known technique involving electrophoresis in small bore capillaries. This approach provides methods for efficient analytical separations of ionic species including macromolecules. A typical capillary electrophoresis system is shown in FIG. 1. As can be seen, a capillary is positioned between two solvent reservoirs containing electrolyte. Electrodes present in each of the reservoirs and coupled to a power supply capable of delivering upwards to 30 kV per 100 cm of capillary provide a voltage gradient to drive charged species through the capillary bore. A detector is positioned at a point between the two high voltage electrodes to permit detection of various ionic species migrating in the capillary. A detector so positioned is sometimes referred to as an on-column detector.
A number of approaches have been developed to detect the solutes separated by capillary electrophoresis depending, in part, upon the nature of the solute detected. UV absorption and fluorescence have been the most commonly used detection modes. Mass spectrometric, radiometric and electrochemical methods of detection have also been utilized. With regard to electrochemical detection, amperometry and conductivity methods have been used.
Off-column amperometric detection utilizing a porous glass capillary to cover a crack in the capillary has also been reported. Wallingford, R. A., et al., Anal. Chem. (1987) 59, 1762-1766; Ewing, A. G., et al., Anal. Chem. (1989) 61, 292A-303A. End-column amperometry and conductivity detection have also been performed. Huang, et al., Anal. chem. (1991), 63, 189-192. A significant problem with on-column and end-column detection utilizing electrochemical techniques is the effect of the high voltage applied to the electrodes to generate the voltage gradient across the capillary. Small variations in this high voltage gradient (typically ranging from 20-30 kV) have significant impact upon the voltages used for amperometric detection and conductivity detection which generally utilizes less than 1 volt in such detection systems.
Even for off-column electrochemical detection, a significant signal to noise problem exists due to the presence of relatively high concentrations of the electrolyte needed in the eluent to generate the voltage gradient across the length of the capillary bore. This is especially problematic for detection utilizing conductivity since such measurements are more significantly affected by electrolyte concentration than amperometric detection which is based primarily upon the detection of redox reactions at the detection electrodes.
Although electrolyte suppression has been used primarily in ion exchange chromatography in conjunction with conductivity detection (see e.g. U.S. Pat. Nos. 3,897,213 3, 3,920,397, 3,925,019, 3,956,559, 4,474,664, 4,751,004, 4,459,357 and 4,999,098), such suppressors have not been adapted for use with capillary electrophoresis.
The references discussed above are provided solely for their disclosure prior to the filing date of the present application and nothing herein is to be construed as an admission that the inventor is not entitled to antedate such disclosure by virtue of prior invention.